A new 6H-type perovskite Ba3Fe2SbO9 has been synthesised for the first time. Synchrotron and neutron powder diffraction data reveal complete structural disorder between Sb and Fe in the octahedral perovskite B sites. This results in classical spin-glass behaviour, which we characterise using magnetic susceptibility, magnetisation, and heat capacity measurements, although some evidence is seen for a transition to a partially ordered spin-glass like state below 24 K. The behaviour of Ba3Fe2SbO9 is compared with that of the 6H-type perovskite Ba3Fe2WO9, which displays antiferromagnetic character below TN = 290K before entering a glassy state below Tf = 60 K. Differences between the magnetism in these two phases are discussed in terms of the complete structural disorder between the Fe and Sb ions in the former case, versus partial disorder (limited to the distribution and local orientation of Fe-W and Fe-Fe dimer units) in the latter.